Abstract Passive
acoustic discrimination of submarines from surface clutter continues to be an important
problem facing the U.S. Navy's anti-submarine warfare (ASW) community. Traditional
approaches rely heavily on the operational instincts of the trained sonar analyst applied
to acoustic data visualization tools in the form of bearing-time records (BTRs) and
lofargrams. However, as sonar system throughput continues to increase, a premium will be
placed on the development of automated feature extraction and classification algorithms
that cue the analyst and enable efficient scene management. Reliable automation of
surface/submerged discrimination is of paramount importance in the dense-multipath,
clutter-dominated littoral environment.
Over the years, a number of investigators have suggested that the fluctuation statistics
of the received field may possess some capacity to distinguish surface and submerged
acoustic sources. Simanin [1] proposed a statistical test based on probabilistic models
for amplitude fluctuations of ray paths of surface and submerged origin. Wagstaff [2]
proposed a lofargram normalization technique based on the short-time harmonic mean of FFT
magnitude, which is purported to emphasize stable submarine signatures over unstable
surface clutter.
More recently, Premus [3] proposed an approach that combines the fluctuation-based
philosophy with a normal mode representation of the acoustic field in a shallow waveguide
to discriminate source class based on the energy in temporal fluctuations of waveguide
mode amplitudes. The short time-scale scintillation of normal mode amplitudes induced by
target interaction with surface waves was demonstrated to possess distinct
surface/submerged discrimination potential for narrowband acoustic sources in a simulated
littoral waveguide [3]. In this paper, data from the ONR-sponsored shallow water
experiment, SWellEX-96, is used to validate the proposed classification methodology and
illustrate the basic phenomenology. Limitations imposed by imperfect knowledge of sound
speed profile and bottom characteristics, the impact of limited effective vertical
aperture, and the consideration of the joint distribution of mode fluctuations across mode
number and frequency will be discussed.

Sponsored in part by SPAWAR, under Air Force Contract F19628-95-C-0002.
Opinions, interpretations, conclusions, and recommendations are those of the authors and
are not necessarily endorsed by the U.S. Air Force.